1. Field of the Invention
The present invention relates to a graphite heating furnace of which at least a part of a furnace body is formed with a graphite.
2. Description of the Related Art
Conventionally, a carbon resistance furnace is commonly used as a heating furnace for a variety of applications requiring a high temperature. The carbon resistance furnace is an electric furnace that employs a graphite in its heating element, having a feature of easily obtaining a high temperature. The graphite is widely used in the heating furnace as a part besides the heating element, such as a heat insulating member and a packing, due to the characteristics of high resistance to heat, small thermal expansion, as well as easy machining. In the present specification, a heating furnace in which at least of part of the heating furnace, particularly, a furnace body of the heating furnace is formed with the graphite is referred to as a graphite heating furnace.
Typically, the graphite heating furnace includes a reaction container in which an object to be heated is placed and a furnace body that encloses the reaction container. Inside the furnace body is maintained in an inert gas atmosphere so that the heating element and the heat insulating member are not degraded from oxidization, and is kept in a positive pressure state with respect to the atmosphere so that the external air does not penetrate into the furnace body. In general, an argon gas is used as the inert gas filled in the furnace body, because the nitrogen gas reacts with the graphite, which generates an extremely poisonous hydrogen-cyanide gas (HCN).
However, from a point of practically view, it is desired to use the nitrogen for the inert gas filled in the furnace body because the argon gas is expensive compared to the nitrogen gas. Therefore, it is required to realize a graphite heating furnace in which the hydrogen-cyanide gas is not generated even when the nitrogen gas is filled in the furnace body. To cope with the problem, a conventional technology has been disclosed for a high-frequency induction heating furnace employing a graphite muffle tube, in which an inner-side portion where the muffle tube is disposed is isolated from an outer-side portion where a high-frequency induction coil is disposed, so that an inert gas other than the nitrogen gas, such as the argon gas, is filled in the inner-side portion, and the nitrogen gas is filled in the outer-side portion (see, for example, Japanese Patent Application Laid-Open No. 2002-173333). In other words, the conventional technology isolates a space in which the graphite is used from a space in which the graphite is not used, and fills the nitrogen gas only in the space in which the graphite is not used.
However, in the conventional technology, a necessity of isolating the spaces with a high airtightness causes another problem that a structure of an isolation wall and the like to realize the isolation performance becomes complicated. In addition, it is still necessary to use the expensive argon gas in the space in which the graphite is used.